Capacity of Quantum Private Information Retrieval with Multiple Servers
This work addresses the efficiency and security of retrieving files from quantum servers without revealing user identity, representing a foundational advance in quantum information theory with potential applications in secure data access.
The authors tackled the problem of quantum private information retrieval (QPIR) with multiple servers, proving that the capacity is 1 regardless of server or file count when servers share prior entanglement, and they constructed a rate-one protocol with two servers that outperforms classical methods in capacity, secrecy, and upload cost.
We study the capacity of quantum private information retrieval (QPIR) with multiple servers. In the QPIR problem with multiple servers, a user retrieves a classical file by downloading quantum systems from multiple servers each of which contains the copy of a classical file set while the identity of the downloaded file is not leaked to each server. The QPIR capacity is defined as the maximum rate of the file size over the whole dimension of the downloaded quantum systems. When the servers are assumed to share prior entanglement, we prove that the QPIR capacity with multiple servers is 1 regardless of the number of servers and files. We construct a rate-one protocol only with two servers. This capacity-achieving protocol outperforms its classical counterpart in the sense of capacity, server secrecy, and upload cost. The strong converse bound is derived concisely without using any secrecy condition. We also prove that the capacity of multi-round QPIR is 1.